Phosphors are currently used in many important devices such as fluorescent lamps, RGB (red, green, blue) screens, lasers, and crystal scintillators for radiation detectors, radiographic imaging, tagging and other security applications, lighting applications, and nuclear spectroscopy. Perhaps the most important property of any phosphor is its brightness, i.e. its efficiency, which is the ratio of the number of optical photons emitted by the phosphor to the energy absorbed. Other important properties include the spectral region of maximum emission (which should match commonly-used photodetectors), optical absorption (minimum self-absorption is desired), decay time of the emission (for some applications fast is desired), and the density. In general, superior scintillators exhibit high quantum efficiency, good linearity of the spectral emission with respect to incident energy, high density, fast decay time, minimal self-absorption, and high effective Z-number. Specific scintillator applications determine the choice of phosphor. Scintillators used for active and passive radiation detection, for example, require high density, and brightness, whereas scintillators used for radiographic imaging also require fast decay time.
An exceptionally good scintillator is cerium-activated lutetium oxyorthosilicate. This material has been conveniently abbreviated in the art as either LSO:Ce or Ce:LSO. LSO:Ce is a crystalline solid that includes a host lattice of lutetium oxyorthosilicate (Lu2SiO5, abbreviated LSO) that is activated by a small amount of the rare-earth (RE) metal dopant cerium (Ce). Cerium is an excellent activator because both its 4f ground and 5d excited states lie within the band gap of about 6 eV of the host LSO lattice. LSO:Ce is very bright, i.e. it has a very high quantum efficiency. LSO:Ce also has a high density (7.4 gm/cm3), a fast decay time (about 40 nanoseconds), a band emission maximum near 420 nanometers, and minimal self-absorption.
While the scintillator properties of LSO:Ce are exceptional, high-quality single crystals are difficult and expensive to prepare. The high cost, which is at least partly due to the high cost of starting materials (high purity Lu2O3 powder) and equipment (iridium crucibles for containing the Lu2O3 powder that melts at about 2150 degrees Celsius), and the tendency of the crystal boule to form cracks that limit the amount of usable single crystal from the boule, limits efforts to develop other types of crystals with an LSO host lattice.
Other exceptionally good scintillators include rare earth doped lanthanum halides, such as cerium-doped lanthanum fluoride, lanthanum chloride, lanthanum bromide, and cerium-doped lanthanum mixed halides. A lanthanum halide host doped with an appropriate phosphor such as Ce(III), for example, is of interest as a scintillator for large-scale radiation detectors.
Light output from nanophosphors has been shown to increase compared to larger phosphors in several systems. In addition, when nanoparticles are used in plastic scintillators, their small size results in reduced light scattering, and hence less attenuation. Nanoparticle phosphors with these properties are less expensive than single crystals to prepare, and could be used to prepare radiation scintillators where detection of radiation over large areas is required. Crystalline lanthanide halide nanophosphors, for example, have attracted recent interest due to their potential uses in optics and optoelectronics (e.g., lighting and displays, optical amplifiers/scintillators, and lasers), microelectronics, and tribology. A particular current interest is their possible use for large-scale scintillators for applications related to homeland security and medical imaging.
Currently, methods for producing micron sized to nanosized particles of rare earth doped or undoped lanthanum halides are limited. Nanoparticles with mean particle sizes below 10 nm of rare earth doped lanthanide oxides, orthosilicates or halides may be prepared using single source precursor, hydrothermal, spray pyrolysis or solution combustion methods (see, for example, Chander in “Development of Nanophosphors—a Review”, Mat. Sci. Eng., vol. R 49, (2005) pp. 113-155, incorporated by reference herein). These particles must then be dispersed in an appropriate medium to prevent agglomeration.
There is a need for better methods for preparing high-quality monodisperse, well shaped, single-crystalline nanoparticles, and also for inexpensive large area, high output radiation detectors.